1. Field of the Invention
The present invention relates to coating compositions based on a polymeric film forming binder and having a sag control agent and particularly to high solids coating compositions based on film forming acrylic polymers and having a sag control agent.
2. Brief Description of the Prior Art
Acrylic polymers are well known in the art as polymeric binders for coating compositions. The acrylic polymers provide for hardness, gloss and exterior durability in the resultant coating making them particularly desirable as finishes for automobiles. The acrylic polymers may be thermoplastic or thermosetting in nature. For thermosetting polymers, the alpha, beta-ethylenically unsaturated monomer component usually includes an active hydrogen-containing monomer such as a hydroxy-containing monomer and the resulting polymer is combined with a curing agent such as a polyisocyanate or aminoplast resin to form a curable or thermosetting composition.
The coating compositions can be formulated with high molecular weight acrylic polymers. However, such polymers require the presence of relatively high amounts of organic solvent to reduce the viscosity of the polymer sufficiently for coatings applications. The high concentration of organic solvent results in air pollution problems. To avoid these problems, there has been a trend in the coatings industry to develop so-called high solids coatings which contain relatively low amounts of organic solvent. The polymers in such high solids compositions are of relatively low molecular weight so that the polymer will have a low viscosity and will not require large concentrations of organic solvent for coatings application. Such high solid compositions, however, have a tendency to sag on application and when baked.
It is disclosed in U.S. Pat. Nos. 3,966,667 and 4,147,688 that such sagging may be reduced by incorporating a microgel polymer in the coating composition. The disadvantage of such treatment, however, is that it may render the cured coating rough in appearance, giving a so-called "orange peel" appearance, particularly on vertical surfaces. Further, the coating containing the microgel polymer may also be milky or yellow in appearance when used in clear coat applications.
U.S. Pat. No. 4,311,622 discloses a crystalline sag control agent which is the reaction product of a diisocyanate and a monamine or hydroxy monoamine. U.S. Pat. No. 4,677,028 discloses a crystalline polyurea sag control agent which is formed by the reaction of an isocyanurate trimer and a primary amine either in situ in the coating composition or externally and then added to the coating composition. While these crystalline sag control agents are reported to reduce roughness in the resultant coating, they may also be visually detectable when used in a clear coat.
It is, therefore, the object of the present invention to provide a sag control agent preferably for clear coating compositions which allows for smoothness on vertical surfaces while maintaining clarity and non-yellowing in the resultant coating.